Field of the Invention
The invention relates to measurement methods for time-of-flight mass spectrometers which operate with pulsed ionization of superficially adsorbed analyte substances and with an improvement in the mass resolution by means of a time-delayed start of the ion acceleration; in particular with ion-accelerating voltages which change over time after a delayed start in order to obtain a rather constant mass resolution over broad mass ranges.
Description of the Related Art
Time-of-flight mass spectrometers are often operated with pulsed ionization of superficially adsorbed analyte substances; methods for the ionization of samples by matrix-assisted laser desorption (MALDI) are known in particular. A plasma cloud, which expands and thus produces a distribution of the velocities of the plasma particles, is generated in the laser focus, said distribution being wider the further the plasma particles (ions and molecules) are from the surface. The velocity distribution means that the mass resolution can be improved by temporally delaying the start of the ion acceleration. Ions of a higher velocity then only pass through a portion of the accelerating field, and thus receive a lower additional acceleration, so the originally slower ions can catch up with them in a temporal focal point. Unfortunately, ions of different mass do not have exactly the same focal point. The focal points for ions of different mass can, however, be made to approach one another if ion-accelerating voltages are used which vary over time after a delayed start, particularly if they continuously increase or decrease (depending on polarity). In combination with a Mamyrin reflector, it is possible to obtain a high mass resolution which is approximately constant over large mass ranges (cf. documents DE 196 38 577 C1, GB 2 317 495 B or U.S. Pat. No. 5,969,348 A, J. Franzen, 1996).
The international patent application WO 2005/114699 A1 describes a standard ion lens system as a corrective ion optic element.